The cheaper plans will make smartphones more affordable to a much bigger market, which in turn should drive bigger unit sales and activations for Apple, Research In Motion, and other companies that sell smartphones at AT&T.

For instance, an iPhone becomes a much more affordable option for kids, lower-end users, and basically anyone who was turned off by the requirement to spend a mandatory $30 per month on data access, whether you used it a lot or a little.

Why will this help sell more phones?

Apple and AT&T have done a lot to reduce the up-front cost of the iPhone since it launched 3 years ago: You can buy an iPhone 3G for $99 after subsidy, down from the iPhone's original 2007 price of $499.

But service costs have remained high: The cheapest iPhone plans available were $40 for voice and $30 for data. Add text messaging fees, taxes, and other fees, and it's easy to see why the average iPhone customer's bill approaches $100 per month. That's obviously much more than most people will spend -- the average U.S. wireless subscriber spends closer to $50 per month for service -- so it was time for carriers to cut rates somehow.

By offering a $15/month entry-level plan, the iPhone and BlackBerry at least SEEM even cheaper than they did before -- even if it's just an opportunity to upsell those subscribers to $25/month plans later on.

For example, A $15/month data subscriber will save $360 over the course of a 2-year contract, or about 20%. The new fees bring the total cost of ownership for a $199 iPhone 3GS buyer (with a $40/month voice plan and a $5/month texting plan) down to $1,640 over a 2-year contract, which is down about 20% from $2,000 previously.

How many more phones will Apple and RIM sell? It's hard to tell: Apple's other pricing policies it may announce at its WWDC conference next week could make an even bigger difference; if the entry-level iPhone drops to $0 after subsidy, for instance.

But by opening the smartphone market to a bigger potential audience, sales figures will point up.